1. Field of the Invention
The present invention relates to a chemical reactor in which a nitrogen oxide or other substance to be treated is subjected to a chemical reaction, and more particularly relates to a chemical reactor with which, for example, nitrogen oxides can be efficiently decomposed and removed from a combustion exhaust gas containing oxygen, and to a method for removing nitrogen oxides by using this chemical reactor.
2. Description of the Related Art
The use of a three-way catalyst is the most common method today for removing nitrogen oxides generated by a gasoline engine. Still, there is a problem in that the adsorption of oxygen that is also present in the gas sharply diminishes the surface activity of the catalyst in diesel engines or under lean burn conditions that afford better fuel economy, and this has led to the use of a removal method featuring intermittent reduction conditions using an oxygen occluding catalyst.
The oxygen molecules in an exhaust gas are also sometimes removed to outside the system without poisoning the surface of the catalyst by using a solid electrolyte membrane having oxygen ion conductivity, and sending electrical current to this membrane. One system that has been proposed as a chemical reactor involves applying voltage to a solid electrolyte sandwiched between electrodes and thereby decomposing the surface-adsorbed oxygen and nitrogen oxides into oxygen and nitrogen, and removing the oxygen from the reaction surface by so-called bumping.
A look at prior publications reveals that J. Electrochemical Soc., 122, 869 (1975) discloses that nitrogen oxides can be decomposed into nitrogen and oxygen by forming a platinum electrode on both sides of zirconia that has been stabilized with scandium oxide and then applying voltage. Also, J. Chem. Soc. Faraday Trans., 91, 1995 (1995) discloses that nitrogen oxides can be decomposed into nitrogen and oxygen in a mixed gas of nitrogen oxides, hydrocarbons, and oxygen by forming a palladium electrode on both sides of zirconia that has been stabilized with yttria and then applying voltage.
A problem with these systems, though, is that a considerable amount of external current must be supplied in order to effect oxygen bumping. Specifically, when oxygen molecules are more likely than nitrogen oxide molecules to be adsorbed to oxygen defects and so forth in the crystal lattice in a reaction, current must be supplied at the outset for removing these occluded oxygen molecules to outside the system, which greatly diminishes the reaction efficiency, something that poses a major obstacle to the practical use of such a system.
To solve the above problem, the amount of oxygen bumping required in a catalyst reaction must be suppressed and higher efficiency achieved in nitrogen oxide adsorption and decomposition reactions in order to minimize the amount of electrical power needed for oxygen bumping.
To this end, there is a method in which the selective adsorptivity to nitrogen oxide molecules is raised at the surface of the catalyst material being used, but so far no material has been obtained that exhibits higher selectivity to nitrogen oxide molecules than to oxygen molecules, and higher reaction efficiency is needed as part of the overall performance achieved by structural control.
In light of the above situation with prior art, the inventors conducted diligent research aimed at developing a novel technique with which the amount of electrical power required by oxygen bumping could be kept to a minimum, and higher efficiency could be attained in nitrogen oxide adsorption and decomposition reactions. As a result, they arrived at the present invention upon discovering that the stated object can be achieved by sufficiently lowering the number of oxygen molecules before they reach the adsorption/removal reaction site, which is accomplished by reducing nitrogen oxides into oxygen ions at the electron conductor in the cathode of the chemical reactor, conducting these ions through an ion conductor, and performing bumping on the anode side.
The present invention provides a chemical reactor that is effective in environmental purification. This is chemical reactor for decomposing and removing a substance to be treated by means of a chemical reaction, comprising an electrochemical cell consisting of a three-layer structure of a cathode, a solid electrolyte, and an anode, in which the internal structure of the cathode is such that nanometer-sized through holes are surrounded by the solid portion, the ion conductor and the electron conductor are distributed in a network form in which they are in close contact with each other, and have a size ranging from the nanometer level to sub-micron. Such a chemical reactor allows for more efficient electrochemical reactions. Also provided is a method for removing nitrogen oxides by using this chemical reactor.
It is an object of the present invention to provide a chemical reactor in which a substance to be treated such as nitrogen oxides is subjected to a chemical reaction.
It is a further object of the present invention to provide a chemical reactor with which nitrogen oxides can be efficiently decomposed and removed from a combustion exhaust gas containing oxygen, and a method for removing nitrogen oxides by using this chemical reactor.
The present invention for achieving the stated objects comprises the following technological means.
(1) A chemical reactor for decomposing and removing a substance to be treated by means of a chemical reaction, comprising an electrochemical cell consisting of a three-layer structure of a cathode, a solid electrolyte, and an anode, wherein the cathode consists of an electrical conductor having nano-sized continuous pores that pass through three-dimensionally.
(2) The chemical reactor according to (1) above, wherein the cathode comprises a material composed of an electroconductive oxide, a metal, a composite of either of these two, or a composite of both of these.
(3) The chemical reactor according to (1) above, wherein the cathode contains as an ion conductor an inorganic solid having oxygen ion conductivity, and contains as an electron conductor an electroconductive metal and/or oxide.
(4) The chemical reactor according to (1) above, wherein the solid electrolyte is an oxygen ion conductor.
(5) The chemical reactor according to (1) above, wherein the pores in the cathode have a diameter of 100 nm or less and are continuously distributed from the surface all the way down to the lower part of the cathode.
(6) The chemical reactor according to (5) above, wherein the pores in the cathode account for at least 10 vol % of the cathode.
(7) The chemical reactor according to (1) above, wherein the continuous pores in the cathode are produced by heat treating the electrical conductor or the electron conductor/ion conductor.
(8) The chemical reactor according to (1) above, wherein the cathode is divided into upper and lower parts, and the lower part of the cathode is both electroconductive and ion conductive.
(9) The chemical reactor according to (3) above, wherein the volumetric ratio of the ion conductor and the electron conductor at the lower part of the cathode is between 3:7 and 7:3.
(10) A method for decomposing and removing a substance to be treated using the chemical reactor according to any of (1) to (9) above, which comprises subjecting a nitrogen oxide to the reactor, applying a current to an electrochemical cell thereof, reducing the nitrogen oxide into oxygen ions by an electron conductor in a cathode, conducting these ions through an ion conductor to lower the relative amount of oxygen gas molecules at the reaction site with respect to the nitrogen oxide gas molecules, and then decomposing the nitrogen oxide.